In a usual facsimile transmission, a sending terminal reads an original image as multi-valued image data by an incorporated scanner, converts the data into binary image data and sends the converted binary data to the receiving terminal, which outputs the received binary image data for printing the data on recording paper.
The conventional image processing device converts multi-valued image data into binary image data in the following way:
First, an original image is divided into character-image areas and halftone (e.g., photographic) image areas.
Means of discriminating between the character image areas and the halftone image areas are usually called as area deciding means.
The area decision is made firstly by examining whether an optical density value of an objective pixel to be examined is within a range between a specified black threshold and a specified white threshold and secondly by examining whether a difference of the density value of the objective pixel from an average density value of four pixels existing in above left, above right, below left and below right of the objective pixel is equal or smaller than a specified threshold value. The objective pixel is decided as a halftone area if said pixel satisfies both conditions as the result of the examinations. The objective pixel is decided as a character area if it does not satisfy both conditions at a time as the result of the examinations.
The halftone pixel thus decided is binarized by a dithering with error diffusion method. A pixel judged as a character area pixel is binarized by using a specified threshold value, and omitting the dithering process, that may impair the quality of character information halftone image represented by halftone dots of a larger size than a certain value of roughness cannot be processed without causing moire fringes by a conventional image processing device. Moire fringes may be produced because the conventional image processing device periodically mistakes an image represented by halftone dots for a character image.
In the conventional image processing device, an area of each of the pixels read by scanning the original image is decided by reference to four pixels existing in just upper, lower, left and right of each objective pixel. In this case, each objective pixel is judged to be of a character area if a difference cyclically exceeds a certain threshold value.
In this halftone dot image, pixels at positions of a certain cycle are misjudged to be of a character area because the differential values of its density from an average density value of the reference pixels exceed the threshold value. These misjudgments cause moire patterns in the image when reproduced.